By way of background, Nuclear Magnetic Resonance (NMR) techniques have been widely used for testing samples. However NMR techniques generally require strong homogeneous magnetic fields to operate, and this adds to the cost and bulk of the equipment required.
NQR testing has been increasingly widely used for detecting the presence or disposition of specific substances. The NQR phenomenon depends on transitions between energy levels of quadrupolar nuclei, which have a spin quantum number I greater than or equal to 1, of which .sup.14 N is an example (I=1). .sup.14 N nuclei are present in a wide range of substances, including animal tissue, bone, food stuffs, explosives and drugs. The basic techniques of NQR testing are well-known and are discussed in numerous references and journals, so will only be mentioned briefly.
In conventional Nuclear Quadrupole Resonance testing a sample is placed within or near to a radio-frequency (r.f.) coil and is irradiated with pulses or sequences of pulses of electro-magnetic radiation having a frequency which is at or very close to a resonance frequency of the quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiant energy will generate an oscillating magnetization which can induce voltage signals in a coil surrounding or close to the sample at the resonance frequency or frequencies and which can hence be detected as a free induction decay (f.i.d.) during a decay period after each pulse or as an echo after two or more pulses. These signals decay at a rate which depends on the time constants T.sub.2 * for the f.i.d., T.sub.2 and T.sub.2e for the echo amplitude as a function of pulse separation, and T.sub.1 for the recovery of the original signal after the conclusion of the pulse or pulse sequence.
A problem initially encountered in NQR techniques was that response signals were often not as easy to process as NMR response signals. In particular, NMR response signals generally exhibit a sinusoidal dependence on flip angle (which is in turn dependent on excitation pulse amplitude and duration); this facilitates processing and combination of signals excited under different conditions. In particular, the observed signal should be exactly inverted after a flip angle of 180.degree., and returned to its initial value after a further 180.degree.. In contrast, NQR signals had been found to exhibit Bessel function dependencies (the order of the Bessel function depending on the nuclei) on flip angle, so that the relationship between detected signal and flip angle, and in particular between signals produced under differing conditions is less straightforward.
In addition, the precession of magnetisation in NMR (as opposed to the transverse oscillation of magnetisation in NQR) allows other useful techniques to be employed, an example of which is the so-called DANTE technique in which a sequence of short pulses separated by short periods of free precession are employed for fine tuning of resonance frequency. This technique, described on pages 207-215 of "A Handbook of Nuclear Magnetic Resonance" by R. Freeman (Longman 1987) is explained in terms of precession based on the rotating frame model, which is conventionally considered inapplicable to NQR.
A number of techniques have been developed to improve measurement of an NQR response. For example, Ramamoorthy et al. (Z. Naturforsch. 45a, 581-586, 1990) have proposed a composite pulse sequence comprising two or three pulse components for compensating inhomogeneity. That disclosure, which recognises that NMR techniques cannot be directly applied to NQR investigations (see particularly page 582, left-hand column), states that "Unlike NMR, it is not feasible to use a large number of pulses within a composite pulse in NQR." The treatment is limited to at most three pulses of carefully chosen phase and duration. A similar proposal has been made by Ageev et al. (Molecular Physics, 1994, Vol. 83, No. 2, 193-210), which also deals with three-component pulses, and affirms the view that "long pulse sequences are not desirable for NQR" (see page 193, second paragraph).
The cancellation of noise and spurious signals has been identified as an important issue in NQR; in this regard, U.S. Pat. No. 5,365,171 (Buess et al.) recognises the problem that "the cancellation techniques conventionally used for NMR are not directly applicable to NQR" and "there is no RF pulse that inverts the entire NQR `magnetisation`" (see particularly column 2). Hence, there have been numerous attempts to provide alternative methods of noise and spurious signal cancellation in NQR; Buess discloses a method of reducing spurious NQR signals in which a pair of sequences of spaced apart (by approximately 1 millisecond) pulses is employed, one member of the pair being of alternating phase the other being of the same phase. Data sampled between each pulse are processed.
Composite pulse sequences comprising up between 3 and 8 mutually different pulse elements have been used to excite broadband resonance, as discussed in J. Mag. Res. A 104, 203-208 (1993). 8 pulse composite sequences are also disclosed in Chem. Phys. Lett. 202, 82-86 (1993).
The applicant's International Patent Application Number WO 96/26453, unpublished at the priority date of this application, discloses a number of phase cycling techniques and pulse sequences which have been found to give particularly good results in eliminating spurious signals.
However, there is still room for improvement in the excitation and detection of NQR responses. Most notably, the efficient inversion of magnetisation which has been so useful in NMR has not been achieved in an NQR experiment. Moreover, as Buess (see above) states, full inversion has been considered impossible in NQR and so research has concentrated on alternative methods of spurious signal cancellation.